20 AH LiMnO2 "overheating" drawing 20 amps??

[doubledipsoon]

ElectricRider described their BMSs as a "Smart high current LiMnO2 24v 7s SOA". This is the first Lithium battery I've ever owned and I don't know much about them other than generic info like protecting the battery from heat, overcharge, undercharge etc.....I contacted ElectricRider to find out if this particular battery is supposed to be fully discharged around 23.6 volts. Since the battery is has a 20 Amp-Hour capacity, my 54 minute ride drawing 20 amps comes out to about that. (18 miles @ 3minutes/mile @drawing 20 amps continuous= about 20AH, right?) So, I'm asumming that when it cut out at 23.6v, the battery was theoretically discharged....I mean, are 24v LiMnO2 batteries "empty" at 23.6 volts?

 

[wesnewell]

It kicked off at right under 23.6v....Now is that the voltage a 24v LiMnO2 battery is supposed to kick off at?

Batteries don't ever kick off until they are drained to 0V. That's why controllers have a low voltage cutoff (LVC). For a 24V system, controller LVC should be 21-22V for lead batteries. It's likely your battery pack has a bms. A bms usually shuts down when a single cell drops below a certain voltage. I have no idea what your bms LVC is. For NMC my guess would be it's somewhere between 2.5-3.0V. Once a cell drops below the set voltage, the whole pack shuts down, regardless of the condition of the other cells. My calcs say 23.6V would be a good voltage to shut it down at. If all cells are balanced, that would be 3.37V per cell. Pretty close to 100% discharged. Now if your cells are out of balance badly, you could have one cell at 2.5V and the others at 3.6V, thus losing range because of the imbalance. Only way to tell is to take individual cell readings.

 

[dogman dan]

100% discharge of a lithium cell is far above 0v. So 23v cutoff is not early at all for your pack.

Below 3.5v, there is very little % of discharge left at all, so when yours cuts off with an average of 3.37v per cell, it's not a bit early. But it doesn't work like that. Your bms will shut off, ( not the overtemp but the lvc shutoff) when any one cell gets too low, like below 3v. So you could be at cutoff early because of one cell is out of balance enough.

But if you can ride a full hour, you are getting all you'd expect from a pack of the size you have, less than 500wh.

GET A #&%&$*(^ Wattmeter. You could have had one at your house weeks ago for under 20 bucks. If you see 400 or more watt hours discharged before she pops, you are good. If she blows at 250 wh, then you are tripping way early. You have a theoretical 450-500 wh.

Measure the watt hours you are getting, find out what your real world max wh is, and then you will always know when something unusual is going on with your range. You'll see, " Oh, it's just the weather or the route today that caused me to run out miles early". Or, "WTF, I only have 200 wh today."

Edit, Pardon me for having less patience this week. Lotta stress watching dad die. But seriously, get a wattmeter. It will help you learn to ride using less than 20 amps continuous.

 

[doubledipsoon]

ElectricRider informed me that when any cell drops below 3.1v the battery engages in "low voltage cutout".....today I test my LiMnO2 battery "open to the air", but on my particular route that I take 3X a week- 8 miles of flat terrain @drawing 20 amps continous 9in the C-1 range, no peddling, fully throttled, with the last mile or so uphill, mild to moderate, with a few short and steep inclines, drawing 25-38 amps in the C-2 range.....I don't anticipate any problems, since yesterday's flat base-line test produced 18 miles @20 amp draws. I'll be monitering the temperature of the battery by touch.......Since it'll be 75 degress here on the coast in the south bay, it should be a primo ride....if this test works out, I'll be cutting vents in my plastic case to provide plenty of laminar air flow past the battery, and test again....

 

[doubledipsoon]

My 9 mile test trip, that is 8 flat miles and 1 uphill mile, went smooth- at the end of the trip the battery was slightly warm to the touch. As my first 18 mile flat test showed, I am getting the full 20 AH, and this particular test has shown that a 1 mile uphill test drawing 25-38 amps did not cause the battery to warm up excessively at all. I think that ElectricRider's claim at delivering a "2C rating" is true, at least for 3 minutes uphill. The voltage at the end of the 9 mile test run (8 flat, 1 uphill) was 25.4v, about at the end of the 10th mile on the "flat" test run.....I'll be cutting 4 holes in the front of the case, 3 at the top, 3 at the bottom, and 4 at the back-end- this should leave to room for heat buildup. It's probably overkill, but it sure won't hurt.

 

[dogman dan]

Yep, you'd have had a noticeably hot battery if it couldn't handle that short bit of 2c continuous.

But learning to get up that hill on less than 2c will save you money in the long haul. Does it, and likes doing it are not the same.

Glad to hear it was just the bms not getting air. It won't take a lot of vent holes to cool the bms.

Electric Rider remains on my trusted battery vendor list. I still will recommend at least 20 ah for a 30 amps controller, but a decent battery from a USA vendor that provides customer support is good to know about.

Thank you very much for providing us your story. It's how we learn to sort the good from the sketchy.

 

[doubledipsoon]

You'll get a big kick out of my newly vented plastic "Currie e-ride" battery case. Hey, why not leave zero possibility for future battery/BMS overheating? Anyway, here's a photo of what happened after I pulled out a drill, a skill saw, and couple of wood drill bits, and made damn sure that things will go right keeping this LiMnO2 cooled down. After I cover the case with some black netting, it'll be not only functional, but very cool to look at.

 

[fechter]

That will reduce the weight a bit too. :wink:

 

[dogman dan]

Definitely should be enough.